1. Field of the Invention
The present invention relates to an image pickup device and an image pickup system.
2. Description of Related Art
In recent years, a higher-image quality inexpensive digital camera has become popular owing to a progress in the performance of the image pickup device. In particular, a CMOS sensor, which has an active element in each pixel and peripheral circuitry on the chip, has remarkably improved its performance, and as a result, CCD sensors have partially been replaced by CMOS sensors. The CMOS sensor has an active element for converting electric charges into an optical signal output in each pixel. The threshold variation of each pixel, and kTC noise (thermal noise) at a time of resetting causes fixed pattern image noise and random image noise. For removing these types of noise, correlated double sampling (CDS) for reading only image signals by acquiring the difference between a reset noise output after resetting and an output after a charge transfer has been proposed.
In the following passage, a problem at the time of performing photography using a CMOS sensor performing the CDS is described. When a very bright light source is photographed in a photographing region, strong light irradiates the electric charge conversion part of the CCD sensor. Consequently, the reset noise output varies owing to the light, and the dynamic range of the active circuit is suppressed. As a result, the image signal of the pixel irradiated by the strong light is reduced (hereinafter this phenomenon is referred to as an image signal reduction at the time of a large quantity of light). For example, when the sun is photographed, the center part of the sun becomes a black point and becomes an unnatural image. This problem is solvable when taking a still image by providing a mechanical shutter. However, in a movie, because the use of the mechanical shutter is disadvantageous for securing an adequate exposure time and a frame speed, a mechanical shutter is not often used to solve this problem. Moreover, because an inexpensive camera frequently omits the mechanical shutter, the problem also occurs even at the time of photographing a still image. In view of such problems, a method of suppressing the optical signal output reduction at the time an image containing a large quantity of light is incident on an image pickup device has been proposed.
Japanese Patent Application Laid-Open No. 2000-287131 proposes a method of detecting an output variation at a time of reading a reset noise to write a predetermined value as a reset noise output when the output variation is judged to be caused by a large light quantity. According to the proposal, at the time of reading an output after a charge transfer, an image signal output reduction prevention circuit is in a cutoff state at the time of receipt of the large light quantity, and does not especially affect an image.
However, the image region other than the pixel which is irradiated by a strong light with a large light quantity is sometimes influenced by the light. FIG. 3 is an explanatory diagram of a CMOS sensor. Each of reference numerals 301 to 303 denotes a unit pixel cell, and is arranged in two dimensions. Reference numerals 304 and 305 denote constant current sources provided to each column. The constant current sources 304 and 305 constitute source follower amplifiers together with the source follower transistors in the pixel cells 301 to 303. A common gate voltage 307 is given to the constant current sources 304 and 305, and a common power source wiring 306 is connected to the constant current sources 304 and 305. A signal of each of the pixels 301 to 303 is read from output terminals 308 and 309 at each row.
When a strong light having an intensity equal to or more than a saturated output irradiates the pixel 302, the voltage of the output terminal 308 falls, and it deviates from the working range the constant current source 304. As a result, a predetermined current is led not to flow through the constant current source 304, and the current quantity flowing through the power source wiring 306 decreases. The constant current sources 305 of the other columns are influenced by the variation of the current, and the voltages of the output terminals 309 are varied to influence the image.
The influence is described with reference to the schematic view of FIG. 4 at the time of a window chart image pickup. A reference numeral 401 denotes a dark output region or not saturated output region, and the dark output region 401 corresponds to the pixels 301 of FIG. 3. A light having an intensity equal to or more than the saturated output irradiates a region 402 corresponding to the pixel 302 of FIG. 3. A reference numeral 403 denotes regions that are irradiated by the same light as that irradiating the regions 401, and the regions 403 correspond to the regions 303 of FIG. 3. An image shaped in a stripe in a lateral direction in the regions 403 is formed under the influence of the saturated region 402.